Many reports have appeared on the attempted specific targeting of tumor cells with monoclonal antibody-drug conjugates (Sela et al, in Immunoconjugates 189–216 (C. Vogel, ed. 1987); Ghose et al, in Targeted Drugs 1–22 (E. Goldberg, ed. 1983); Diener et al, in Antibody mediated delivery systems 1–23 (J. Rodwell, ed. 1988); Pietersz et al, in Antibody mediated delivery systems 25–53 (J. Rodwell, ed. 1988); Bumol et al, in Antibody mediated delivery systems 55–79 (J. Rodwell, ed. 1988). All references and patents cited herein are incorporated by reference.
Cytotoxic drugs such as methotrexate, daunorubicin, doxorubicin, vincristine, vinblastine, melphalan, mitomycin C, and chlorambucil have been conjugated to a variety of murine monoclonal antibodies. In some cases, the drug molecules were linked to the antibody molecules through an intermediary carrier molecule such as serum albumin (Garnett et al, 46 Cancer Res. 2407–2412 (1986); Ohkawa et al 23 Cancer Immunol. Immunother. 81–86 (1986); Endo et al, 47 Cancer Res. 1076–1080 (1980)), dextran (Hurwitz et al, 2 Appl. Biochem. 25–35 (1980); Manabi et al, 34 Biochem. Pharmacol. 289–291 (1985); Dillman et al, 46 Cancer Res. 4886–4891 (1986); Shoval et al, 85 Proc. Natl. Acad. Sci. 8276–8280 (1988)), or polyglutamic acid (Tsukada et al, 73 J. Natl. Canc. Inst. 721–729 (1984); Kato et al 27 J. Med. Chem. 1602–1607 (1984); Tsukada et al, 52 Br. J. Cancer 111–116 (1985)).
A wide array of linker technologies has been employed for the preparation of such immunoconjugates and both cleavable and non-cleavable linkers have been investigated. In most cases, the full cytotoxic potential of the drugs could only be observed, however, if the drug molecules could be released from the conjugates in unmodified form at the target site.
One of the cleavable linkers that has been employed for the preparation of antibody-drug conjugates is an acid-labile linker based on cis-aconitic acid that takes advantage of the acidic environment of different intracellular compartments such as the endosomes encountered during receptor mediated endocytosis and the lysosomes. Shen and Ryser introduced this method for the preparation of conjugates of daunorubicin with macromolecular carriers (102 Biochem. Biophys. Res. Commun. 1048–1054 (1981)). Yang and Reisfeld used the same technique to conjugate daunorubicin to an anti-melanoma antibody (80 J. Natl. Canc. Inst. 1154–1159 (1988)). Dillman et al. also used an acid-labile linker in a similar fashion to prepare conjugates of daunorubicin with an anti-T cell antibody (48 Cancer Res. 6097–6102 (1988)).
An alternative approach, explored by Trouet et al, involved linking daunorubicin to an antibody via a peptide spacer arm (79 Proc. Natl. Acad. Sci. 626629 (1982)). This was done under the premise that free drug could be released from such a conjugate by the action of lysosomal peptidases.
In vitro cytotoxicity tests, however, have revealed that antibody-drug conjugates rarely achieved the same cytotoxic potency as the free unconjugated drugs. This suggested that mechanisms by which drug molecules are released from the antibodies are very inefficient. In the area of immunotoxins, conjugates formed via disulfide bridges between monoclonal antibodies and catalytically active protein toxins were shown to be more cytotoxic than conjugates containing other linkers. See, Lambert et al, 260 J. Biol. Chem. 12035–12041 (1985); Lambert et al, in Immunotoxins 175–209 (A. Frankel, ed. 1988); Ghetie et al 48 Cancer Res. 2610–2617 (1988). This was attributed to the high intracellular concentration of glutathione contributing to the efficient cleavage of the disulfide bond between an antibody molecule and a toxin. Despite this, there are only a few reported examples of the use of disulfide bridges for the preparation of conjugates between drugs and macromolecules. Shen et al (260 J. Biol. Chem. 10905–10908 (1985)) described the conversion of methotrexate into a mercaptoethylamide derivative followed by conjugation with poly-D-lysine via a disulfide bond. Another report described the preparation of a conjugate of the trisulfide containing toxic drug calicheamycin with an antibody (Hinman et al., 53 Cancer Res. 3336–3342 (1993)).
One reason for the lack of disulfide linked antibody-drug conjugates is the unavailability of cytotoxic drugs possessing a sulfur atom containing moiety that can be readily used to link the drug to an antibody via a disulfide bridge. Furthermore, chemical modification of existing drugs is difficult without diminishing their cytotoxic potential.
Another major drawback with existing antibody-drug conjugates is their inability to deliver a sufficient concentration of drug to the target site because of the limited number of targeted antigens and the relatively moderate cytotoxicity of cancerostatic drugs like methotrexate, daunorubicin, and vincristine. In order to achieve significant cytotoxicity, linkage of a large number of drug molecules, either directly to the antibody or through a polymeric carrier molecule, becomes necessary. However, such heavily modified antibodies often display impaired binding to the target antigen and fast in vivo clearance from the blood stream.
In spite of the above described difficulties, useful cytotoxic agents comprising cell binding moieties and the group of cytotoxic drugs known as maytansinoids have been reported (U.S. Pat. No. 5,208,020, U.S. Pat. No. 5,416,064, and R. V. J. Chari, 31 Advanced Drug Delivery Reviews 89–104 (1998)). Similarly, useful cytotoxic agents comprising cell binding moieties and analogues and derivatives of the potent antitumor antibotic CC-1065 have also been reported (U.S. Pat. No. 5,475,092 and U.S. Pat. No. 5,585,499).
Paclitaxel (Taxol), a cytotoxic natural product, and docetaxel (Taxotere), a semi-synthetic derivative (See FIG. 1), are widely used in the treatment of cancer. These compounds belong to the family of compounds called taxanes. Taxanes are mitotic spindle poisons that inhibit the depolymerization of tubulin, resulting in an increase in the rate of microtubule assembly and cell death. While docetaxel and paclitaxel are useful agents in the treatment of cancer, their antitumor activity is limited because of their non-specific toxicity towards normal cells.
Further, compounds like paclitaxel and docetaxel themselves are not sufficiently potent to be used in conjugates of cell binding agents. Recently, a few new docetaxel analogs with greater potency than either docetaxel or paclitaxel have been described (Iwao Ojima et al., J. Med. Chem. 39, 3889–3896 (1996) and FIG. 1). However, these compounds lack a suitable functionality that allows linkage via a cleavable bond to cell binding agents.
Accordingly, a method of treating diseases with taxanes wherein their side effects are reduced without compromising their cytotoxicity is greatly needed.